The abrupt resignation of BP chief Bernard Looney marks the loss of an executive who pushed for a transition to clean energy more aggressively than any of his industry peers, with mixed results.

BP said Tuesday that the 53-year-old CEO was departing after failing to fully disclose to the company board some of his past relationships with colleagues. Chief Financial Officer Murray Auchincloss will take the top job on an interim basis.

Looney’s exit caps a tumultuous three years leading BP in its most radical strategy shift since John Browne transformed the company into a transatlantic giant by buying U.S. rivals Amoco and Atlantic Richfield more than two decades earlier. From the North Sea to West Texas, Looney pushed BP into greener territory, with big bets on hydrogen and offshore wind. Without the architect of that pivot, the company’s direction is now in question.